In recent years, due to explosive development and widespread of computers and their networks, a variety of information such as character data, image data and voice data have been digitized. Digital information does not undergo degradation due to secular changes and thus can be stored in perfect state permanently, but it can be easily duplicated and therefore the protection of copyright becomes an important issue. For this reason, security technologies for protecting copyright assume greater importance at a rapid pace.
One of techniques for protecting copyright is an “digital watermark”. The digital watermark is a technique of embedding the name of a possessor of copyright and the ID of a purchaser in digital image data, voice data, character data and the like in a form imperceptible by persons, and tracking unauthorized use thereof by illegal copying.
The digital watermark may be subjected to variety of attacks. They include, for example, irreversible compression such as JPEG, geometric transformation such as scaling or rotation, printout/scanning and noise attachment.
Here, a process called registration may be used particularly for providing resistance to geometric transformation. Registration is a process of adding a specific signal (registration signal) to an image in addition to added information at the time of embedding the digital watermark, and promoting the extraction of added information using the above described registration signal before added information is extracted, at the time of extracting added information.
Conventionally, the registration signal having a symmetric axis is embedded when an digital watermark is embedded, whereby the image with the digital watermark embedded therein can be subjected to frequency conversion to analyze the scaling and rotation applied to the image.
However, when rotation applied to the image is analyzed, the direction of the rotation applied to the image with the digital watermark embedded therein cannot be uniquely determined due to the symmetry in the amplitude spectrum of the spatial frequency domain.